In the field of fluid control various valves have been used to control the flow of the fluid. Well known in the field are two position valves which are either open or closed and proportional valves where the amount of fluid flowing is determined by the degree of openness or closure of the valve. Also known in the field are valves where the fluid flows directly through the valve and pinch valves where the valve operates on the external portion of the flow path to control the flow of the fluid along a flow path. An example of this latter case is a roller clamp used in the intravenous delivery of therapeutic solutions where the roller pinches the IV flow line based on its position along an inclined plane.
Also known in the art are bistable latching valves that may either be open or closed. These bistable latching valves are of particular importance when available power for operation of the valve is limited. In the case of these bistable latching valves power need only be applied to change the state of the valve; that is, from open to closed or from closed to open. Since these valves are stable in both the open position and the closed position, no power is needed to keep the valve in either the open or the closed position. Examples of latching valves are commercially available from the Lee Company of Westbrook, Conn.
Most latching valves are designed in such a manner that fluid flows through the valves. However, latching pinch valves are known, for example, solenoid latching pinch valves from the Farmington Engineering Company of Madison, Conn.
Valves taking advantage of the shape changing properties of shape memory alloys are also known. Krumme in U.S. Pat. No. 4,645,489 teaches the use of a shape memory allow to control the position of a valve closure element in a proportional valve. Edelman and Ritson in U.S. Pat. No. 4,878,646 teach the use of a shape memory alloy element to release the energy of a spring to close a pinch valve in an IV fluid delivery system. In this teaching, the shape memory alloy is only used to close the valve by releasing a latch. Reopening the valve and latching the valve open is done manually. In automatic fluid control systems, where both the closing and the opening of the fluid flow path should be done automatically under the control of an operating system, such a manual “reset” of the valve of Edelman and Ritson is impractical.
Recently, “closed loop” fluid systems for delivery of medications to patients have appeared where a fluid flow property is measured and the rate of fluid flow is adjusted based on the flow measurement. Notable examples are Sage, in U.S. Pat. No. 6,582,393, Connelly et al in U.S. Pat. No. 6,589,229 and Jerman in U.S. Pat. No. 5,533,412. While Connelly uses a piezoelectric pump and adjusts the output of the pump based on the monitored flow property, Sage and Jerman do not teach the details of the flow control means, although both do teach the flow measuring means. An automatic, low power, bistable latching pinch valve would be useful in the implementation of either the art of Jerman or Sage. None of the prior art teachings represent acceptable valves for use in these miniaturized fluid delivery systems where the fluid flow rate is measured and a microprocessor controls a valve based on the measured flow rate. Hence there remains a need for improved valving methods.